The present invention relates to an arrangement of a reference frequency oscillation source, which is used in a base station with high frequency precision and terminals for radio communications, capable of reducing variations in oscillation frequency due to its temperature changes or secular changes. The present invention also relates to a method for controlling the novel arrangement.
The oscillation frequency of a crystal oscillator is decided by the inherent frequency of the piezoelectric crystal resonator contained therein. The piezoelectric crystal has its stable inherent oscillation frequency and can advantageously provide a stable oscillator. This crystal element is particularly used as a time or frequency reference source in radio communications apparatuses.
In recent years, large capacity communication systems have been developed to respond to an increasing number of users of radio communications typically including cellular telephones. Such large capacity communication systems require oscillators with a frequency error of, for example, less than 1 ppm. There are strong demands for time or frequency reference sources with higher precision. Particularly, the direct sequence code division multiple access (DS-CDMA) system that basically adds receive signals in phase is used to realize excellent noise-resistance and anti-interference. However, an error in frequency between a transmitter and a receiver causes the addition in phase to fail, thus resulting in remarkable characteristic degradation. In general, the temperature compensation crystal oscillator (TCXO) for a frequency standard using a simple analog technique can realize a frequency precision of about 3 ppm. However, in the case of the DS-CDMA system, since the noise-resistance and anti-interference characteristics can be advantageously improved by increasing the number of symbols added in phase, the frequency reference with a high precision of less than 1 ppm is sometimes needed. In that case, the temperature compensation in an analog fashion leads to a complicated circuit configuration, thus increasing the manufacturing costs and the space for assembly. To solve such problems, JP-A 5605/1990, for example, discloses the method of storing data compensating the temperature characteristic indicating a frequency change to a frequency control signal of a crystal oscillator into a ROM, instead of a simple temperature compensation in an analog fashion, and then performing the digital temperature compensation based on the temperature measured with a temperature sensor.
FIG. 3 illustrate the configuration of the conventional digital temperature compensation crystal oscillator disclosed in JP-A 5605/1990. Referring to FIG. 3, a temperature sensor 1 measures the ambient temperature of the voltage controlled crystal oscillator 9. An address creating unit 2 creates an address at which corresponding compensation data is stored. A read-only memory (ROM) 3 previously stores compensation data over the entire temperature range for each crystal oscillator and outputs the compensation data for an address designated by the address creating unit. Successively, the D/A converter converts compensation data output from the ROM 3 into an analog signal. The resonator circuit 7 is controlled with the analog signal to compensate the oscillation frequency of the oscillator 8. Finally, the oscillator 8 outputs the reference frequency signal 800. The prior art publication further discloses the method of compensating the oscillation frequency by means of plural pairs of an analog switch and a capacitor, without using the D/A converter 6. The detail is not described here.
On the other hand, the automatic frequency control (AFC) is listed as a method of compensating errors in frequency between a transmitter and a receiver using the receive signal on the receiver side. There are various AFC methods. The JP-A 79145/1996, for example, discloses the frequency compensating method using two receiving systems. One desired wave extracting filter is just set to the signal band to be received while the other desired wave extracting filter is set to a broader band containing a variation in center frequency due to a frequency error between the transmitter and a receiver plus the signal band to be received. Thus, by preventing the attenuation of a receive signal power due to the desired wave extracting filter at the presence of a frequency error, the frequency pulling operation can be performed under the AFC even if the frequency error is large.
Moreover, JP-A 245563/1995 discloses another frequency compensating method. According to this method, the frequency offset data for several patterns stored in a ROM are swept on trial at the time of starting the AFC while a frequency error between a transmitter and a receiver is measured. Trial and error is repeated with various types of frequency offset data until the frequency error falls finally within the range in which the normal AFC operation can be executed.
The configuration disclosed in JP-A 5605/1990 performs only the step of compensating oscillation frequency variations due to temperature changes. Since the frequency error between a transmitter and a receiver has a relative value, the transmitter side with a frequency precision may sometimes need AFC to compensate the frequency error therebetween.
In the DS-CDMA system, the receiving operation is started with a certain frequency error (e.g. less than 1 ppm) between a transmitter and a receiver. Then, the relative error to the frequency of the transmitter must be controlled to a smaller value (e.g. less than 0.1 ppm) by performing the AFC operation using the receive signal received by a receiver.
On the condition that the frequency precision of a transmitter is so high that the frequency error can be ignored to an actual frequency, there is no problem if the frequency compensation of the configuration disclosed in JP-A 5605/1990 can improve the frequency precision of a receiver so as to ignore the frequency error to the actual frequency. However, it is difficult to actually satisfy such a condition because the amount of compensation data stored into the ROM 3 increases and the precision of the temperature sensor 1 is insufficient.
Even if the compensation of a variation in oscillation frequency due to temperature changes is merely combined with the AFC operation by a receive signal, the oscillation frequency varied due to ambient temperature changes is compensated during the AFC operation by the receive signal. As a result, the high precision frequency compensation under the AFC by the receive signal may be carried out in vain.
In addition, the frequency variation due to aging may cause a frequency error when the frequency variation caused by a temperature change is compensated with a preset compensation value, so that the receiving operation cannot be performed under the worst conditions.
According to, the configuration disclosed in, for example, JP-A 79145/1996, when there is a frequency error between a transmitter and a receiver, the desired wave extracting filter with the filtering characteristics fully matched to the band of a signal to be received attenuates its output. The output of the desired wave extracting filter with a slightly broader band contains undesired band noise components. Hence, the S/N ratio of the configuration is deteriorated. Moreover, in the DS-CDMA system, when the frequency error is large to some extent, the receive signal cannot be demodulated, so that even the AFC control cannot be performed.
According to JP-A 245563/1995, the configuration decides on trial the compensation amount for a frequency error without using information from the temperature sensor. Hence, a long period of time is taken to begin the frequency pulling operation under the AFC operation.